1. Field of the Invention
The present invention relates generally to methods and apparatus for generating a plasma at or about one atmosphere, especially for purposes of treating various webs and films to enhance their properties, and to the treated webs and films, which have the improved and desirable properties.
2. Description of the Related Art
The surface treatment of polymer materials using a plasma discharge can lead to a broad range of improved results. A plasma discharge can be used to initiate chemical reactions on the surface of a substrate or roughen a surface from ion bombardment. One important benefit that can be achieved is to provide a more hydrophilic, or wettable surface. Plasmas produced under a vacuum have produced hydrophilic surfaces. However, this effect is typically short term for vacuum plasma treated materials. Recent experiments using a one atmosphere dielectric barrier discharge, with a sinusoidal excitation of a few kilohertz, have produced meltblown polypropylene samples which were wettable for eight months, and longer. However, the treatment times for these samples were generally on the order of four to five minutes, which is considered relatively long for practical applications.
By controlling certain processes of the plasma/substrate interaction, and by exploiting various features associated with a one atmosphere discharge, higher plasma power densities and shorter treatment times can be obtained. When exposed to a plasma, a substrate will be bombarded by electrons, ions, radicals, neutrals and ultraviolet (UV) radiation which is sometimes sufficient to cause sputtering or etching of the exposed surface. The resulting volatile products are likely to contaminate the working gas and can be redeposited on the substrate. Sufficient gas flow within the discharge zone can minimize these problems. However, in addition to etching and roughening the substrate, ions can react chemically with the substrate.
The energy and flux of ions to the substrate can be significantly increased by biasing the substrate, usually to a negative potential. Controlled substrate biasing for a high pressure discharge requires metal faced electrodes or an asymmetric voltage waveform when using a dielectric barrier discharge. A symmetric or sinusoidal waveform will alternately bias a substrate positively and then negatively throughout a cycle, partially reversing the effects produced by each half cycle.
The energetic UV radiation produced by a plasma can have a variety of effects on both the background gas and the polymer substrate. Vacuum UV (primarily at short wavelengths, typically 50 to 250 nm) can cause photoionization and bond dissociation yielding free radicals. Radicals produced on a polymer surface can cause crosslinking of a polymer chain or react with species present in the gas phase. For the production of a hydrophilic surface, oxygen or oxygen containing radicals must typically be present. Since many competing reactions will occur in an oxygen containing gas phase, and since these reactions will have temperature dependent reaction rates, proper control of the background gas temperature will result in higher concentrations of the appropriate species to enhance a given surface treatment.
Ultraviolet production in a gas phase discharge can be enhanced by the use of a gas with accessible emission lines (in the UV) for the operating mode of the discharge. Proper electrode geometry with metal faced electrodes reflective to UV, and a dielectric barrier transparent to UV, will enhance the UV levels in the gas discharge.
It is therefore the primary object of the present invention to provide for the improved treatment of webs and films, especially those formed of polymer materials, with a plasma generated, for example, at or about one atmosphere of pressure and in a relatively short period of time.
It is also an object of the present invention to provide webs and films, especially those formed of polymer materials, which have been treated with a plasma generated, for example, at or about one atmosphere of pressure to enhance their properties, especially in terms of their wettability (hydrophilicity) or non-wettability (hydrophobicity).
It is also an object of the present invention to provide improved methods for treating such webs and films to enhance their properties, especially in terms of their wettability (hydrophilicity) or non-wettability (hydrophobicity) as well as other desirable properties such as printability, especially for films.
It is also an object of the present invention to provide methods for treating such webs and films to achieve the foregoing improvements, which exhibit relatively short exposure times while avoiding the potential for damage to the substrate which is to be treated.
It is also an object of the present invention to provide apparatus for implementing the foregoing methods, for the treatment of webs and films to suitably enhance their properties.
It is also an object of the present invention to provide electrode designs for implementing the foregoing methods.
It is also an object of the present invention to provide corresponding circuit designs for suitably exciting the electrodes of the present invention.
These and other objects which will become apparent are achieved in accordance with the present invention by two methods and corresponding electrode designs for the generation of a plasma, for example, at or about one atmosphere.
A first method utilizes a repetitive, asymmetric voltage pulse to generate a plasma discharge between two electrodes. An asymmetric voltage pulse is used to generate a discharge in which a substrate can be exposed predominately to either positive or negative plasma species depending on the voltage polarity used. A second method uses the gap capacitance of an electrode pair and an external inductor in shunt to form a resonant LC circuit. The circuit is driven by a high power radio frequency source operating at 1 to 30 MHz to generate a uniform discharge between the electrode pair.
Both methods have temperature controlled discharge surfaces with supply gas temperature, humidity and flow rate control. The gas flow is typically sufficient to cause a turbulent flow field in the discharge region where materials are treated. Such methods are generally intended to operate within a metal enclosure to allow containment of the working gas and to provide shielding of the electromagnetic fields.
The foregoing methods are preferably practiced with an electrode pair including a metal faced electrode and a dielectric covered electrode, one or both of which have a series of holes extending through the electrode face for supply gas flow. The second of the above-described methods will also operate with paired, metal faced electrodes, but under more restricted operating conditions.
A remarkable aspect of the present invention is that improved properties can be imparted to webs and films within a treatment period which is very short. In accordance with the present invention, improved properties can be obtained by exposure to the plasma in sixty seconds, or less, frequently with treatments of less than 20 seconds, and quite satisfactorily for periods of time as little as 1.5 seconds. When sequential treatments are performed, the above-mentioned times refer to total timed exposure to the plasma.
The invention can be practiced with a variety of gases, typically inert gases like helium and argon, active gases like oxygen and nitrogen, and more complex gaseous molecules like carbon dioxide and ammonia. Gases may be used in mixtures (of two or more gases), including air, or a single gas with oxygen or some other suitable gas. Gaseous mixtures including oxygen are preferably combined in relative proportions including 2 to 20% oxygen. The gaseous mixtures may be essentially dry (i.e., essentially gaseous), or may be biphasic, such as a gas containing relatively limited proportions of a liquid (e.g., water vapor). Additional gases which may be used for appropriate applications would include hydrogen (e.g., for saturating a polymer to create a more hydrophobic surface) and some of the fluorocarbons like CF4.
For further discussion of the improved methods and electrode configurations, and webs and films of this invention, reference is made to the detailed description which is provided below, taken in conjunction with the following illustrations.